1. Field of the Invention
This invention relates to the field of photometry. In particular, the invention relates to spectrophotometric methods and apparatus capable of determining the light absorption pathlength for various samples to be analyzed with a spectrophotometer.
2. Description of the Related Art
The problem of an undefined light absorbance pathlength in vertical-beam photometers has existed since the advent of vertical-beam photometers, i.e., for over 20 years. Substantial errors in determination of either relative optical pathlength or the concentration of analytes in vessels of unknown optical pathlength by prior art methods occur because of 1) substantial variation in solvent temperature, 2) substantial variation in solvent composition and 3) substantial presence of materials in the samples which absorb light in the wavelength region where the optical pathlength of the solvent is being monitored.
Photometry is a common measurement technique employed to monitor optical characteristics of samples. Customarily, samples contain an analyte species dissolved in a solvent at an unknown concentration. The concentration of the analyte may be determined by using a photometric device to measure the fraction of light absorbed by the sample at a specific wavelength (.lambda.). The value of .lambda. is usually chosen to be near the wavelength of light where the analyte absorbs maximally. According to the Beer-Lambert law, equation 1, absorbance is determined as follows: ##EQU1## where I.sub.0 is the incident radiation intensity, I is the intensity of light emerging from the sample, .epsilon..sub..lambda. is the molar extinction coefficient of the analyte dissolved in the solvent, l is the light absorption pathlength, and C is the concentration of absorbing analyte in the solvent. The value of I customarily is measured with a photometric apparatus, such as a photometer or spectrophotometer, equipped with a fixed light path sample-retaining device called a cuvette, such as a 1 cm wide cuvette. The sample-retaining device contains the sample comprised of analyte dissolved in a solvent. The value of I.sub.0 is ordinarily measured with the same system except that no analyte is present in the solvent. Alternatively, I.sub.0 may be measured in the absence of both the sample and the sample-retaining device (this value of I.sub.0 is called an "air blank"). When an "air blank" is employed, a separate A.sub..lambda. measurement of the solvent and sample-retaining device gives a "solvent blank" absorbance value. A "corrected absorbance" value related to absorbance of the analyte is then obtained by subtracting the "solvent blank" from each absorbance measurement made on the samples comprised of analyte dissolved in solvent and contained in the sample-retaining device. These two alternative procedures give mathematically equivalent results. Absorbance measurements made by either procedure allows unknown concentrations of the analyte to be determined by calculation according to Eq. 1, provided that .epsilon..sub..lambda. and l are known.
A spectrophotometer is a photometric apparatus which employs an adjustable means to pre-select a desired portion of the electromagnetic spectrum as incident radiation. Usually spectrophotometers employ a dispersive means such as a prism or diffraction-grating monochrometer to provide continuously selectable, narrow, bands of light centered about the desired wavelength .lambda.. Most conventional photometers and spectrophotometers employ a horizontal light beam that traverses the liquid sample horizontally so as to avoid passing through the liquid-gas interface that is typically above the sample. With such horizontal-beam photometers, the geometry and optical pathlength within the sample is fixed for any given cuvette. For visible and ultraviolet light absorption measurements, for example, cuvettes customarily have a 1 cm pathlength. Cuvettes with pathlengths between 0.1 cm and 10 cm are also common, however. With any such fixed pathlength cuvette, in a horizontal-beam photometers, unknown concentrations C of the analytes may be calculated from absorbance measurements provided that the values of .epsilon..sub..lambda. and l are known.
When either .epsilon..sub..lambda. and l is not known, values C of may be determined readily by employing known concentrations of the analyte dissolved in the same solvent (i.e., standards) and performing similar light-absorbance measurements on the unknowns and on the standards. The most common procedure comprises plotting A.sub..lambda. versus concentration of analyte in the standards (i.e., a "standard curve") and then comparing the results obtained with the unknown concentrations of analyte to the standard curve. This procedure provides the unknown concentrations of analyte from the "standard curve".
Vertical-beam photometers also measure light absorption in order to determine the unknown concentrations of analyte in samples. In vertical-beam, photometers, however, the light beam usually passes only through one wall of the sample-retaining device, through the sample, and through the interface between the sample a surrounding gas atmosphere (which is usually air). The latter liquid-gas interface, the meniscus, is usually curved, the specific shape depending upon the interactions between the liquid sample and the gas and the side-walls of the sample-retaining device. Depending upon the design of a particular vertical-beam photometer, the light beam may traverse the meniscus either before or after passing through the sample. In either case, the optical pathlength through the sample is not a constant value. Instead, the optical pathlength is related to the sample volume and the meniscus shape. The nature of the sample, the sample-retaining device surfaces, and gas each contributes to the shape of the meniscus, quantitatively affecting the optical pathlength through the sample. Thus, in vertical-beam photometers, the value of l in Eq. 1 usually is unknown and difficult to control reproducibly.
Vertical-beam photometry has become a popular technique despite the disadvantage of not having a fixed optical pathlength through the sample. This popularity stems from the fact that the optical characteristics of a large multiplicity of samples may be analyzed with a vertical-beam photometer in a small period of time. Typically, vertical beam photometers monitor the optical characteristics of samples disposed in the wells of, for example, 96 well multi-assay plates. The optical characteristics, such as light absorption or light scattering, of the samples contained within each well of such multi-assay plates may be monitored, typically, in 10 seconds or less. Vertical-beam photometers also allow repetitive measurements of such a multiplicity of samples to be made with intervals of 10 seconds or less between each of a series of measurements. In such a way the kinetic properties, such as the rate of change in absorbance, of a plurality of samples may be monitored in a very short time.
In vertical-beam photometry of the prior art, an approximated constant value of l is used for standards and unknowns. Concentrations of unknown analytes are determined, often with acceptable precision, by plotting "standard curves" using the approximated value of l and comparing the absorbance results obtained with unknown concentrations of analyte to the "standard curve," as mentioned previously.
The fact that a value of C may not be calculated directly from Eq. 1, but instead must be determined from a "standard curve" constructed for each analytical measurement, severely hinders the ability of vertical-beam photometric techniques. The additional time and expense required for preparing such standard curves for each analysis is often an onerous disadvantage to vertical-beam photometry. Thus, convenient, accurate, and precise methods and apparatus for determining optical pathlength of samples in vertical-beam photometers would be of great utility.
Japanese Kokai Patent Application number Sho58 1983! - 1679Y2 discloses the unknown optical pathlength of vessels may be determined by dispensing a colored solution, with a known relationship between optical pathlength and absorbance, into the vessels and determining the absorbance of this solution. A similar method is taught in U.S. Pat. No. 5,298,978, issued Mar. 29, 1994.
Additionally Japanese Kokai Patent Application numbers Sho 60(1985!-183560 and Sho 611986!-82145 disclose methods of determining relative optical pathlength of aqueous samples within different reactor vessels (contained in a common reactor) by measuring the optical density of the samples at two different wavelengths in the near-infrared wavelength region from 900 to 2100 nanometers. With clear quartz reaction vessels, the reference teaches that (A.sub.975 -A.sub.900), A.sub.1195 -A.sub.1070), or (A.sub.1260 -A.sub.1070) may be used to determine the relative optical pathlength through aqueous samples. For reactors made of synthetic acryl resins, where the resin has interfering absorption bands, the prior art teaches that (A.sub.970 -A.sub.1070) or (A.sub.1280 -A.sub.1070) may be used to determine relative optical pathlength of the samples. Once relative optical pathlength is known for each of the vessels of the reactor, then optical density values of an analyte (measured at a third wavelength) may be normalized for variation in optical pathlength to obtain the relative concentration of analyte in each reactor vessel. Employing vessels with known concentrations of analyte allows on to determine the absolute concentrations of analyte within other vessels.
There also exists need for methods and apparatus that may be utilized with samples that are dissolved in a variety of different solvents or in mixtures of different solvents. Because analytes are extremely diverse and may have diverse light-absorption properties, there exist no apparatus capable of determining concentration and optical pathlength of any analyte dissolved in various solvents or mixtures of solvents. Further complicating this situation is the extreme variability of concentrations of analytes from one sample to the next.